331 research outputs found
Ultrafast dynamics in the presence of antiferromagnetic correlations in electron-doped cuprate LaCeCuO
We used femtosecond optical pump-probe spectroscopy to study the photoinduced
change in reflectivity of thin films of the electron-doped cuprate
LaCeCuO (LCCO) with dopings of x0.08 (underdoped) and
x0.11 (optimally doped). Above T, we observe fluence-dependent
relaxation rates which onset at a similar temperature that transport
measurements first see signatures of antiferromagnetic correlations. Upon
suppressing superconductivity with a magnetic field, it is found that the
fluence and temperature dependence of relaxation rates is consistent with
bimolecular recombination of electrons and holes across a gap (2)
originating from antiferromagnetic correlations which comprise the pseudogap in
electron-doped cuprates. This can be used to learn about coupling between
electrons and high-energy () excitations in these
compounds and set limits on the timescales on which antiferromagnetic
correlations are static
Real time observation of cuprates structural dynamics by Ultrafast Electron Crystallography
The phonon-mediated attractive interaction between carriers leads to the
Cooper pair formation in conventional superconductors. Despite decades of
research, the glue holding Cooper pairs in high-temperature superconducting
cuprates is still controversial, and the same is true as for the relative
involvement of structural and electronic degrees of freedom. Ultrafast electron
crystallography (UEC) offers, through observation of spatio-temporally resolved
diffraction, the means for determining structural dynamics and the possible
role of electron-lattice interaction. A polarized femtosecond (fs) laser pulse
excites the charge carriers, which relax through electron-electron and
electron-phonon coupling, and the consequential structural distortion is
followed diffracting fs electron pulses. In this review, the recent findings
obtained on cuprates are summarized. In particular, we discuss the strength and
symmetry of the directional electron-phonon coupling in Bi2Sr2CaCu2O8+\delta
(BSCCO), as well as the c-axis structural instability induced by near-infrared
pulses in La2CuO4 (LCO). The theoretical implications of these results are
discussed with focus on the possibility of charge stripes being significant in
accounting for the polarization anisotropy of BSCCO, and cohesion energy
(Madelung) calculations being descriptive of the c-axis instability in LCO
Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr_2IrO_4 from time-resolved photocarrier dynamics
We perform a time-resolved optical study of Sr_2IrO_4 to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled J_(eff) = 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the Néel temperature T_N the system evolves continuously from a metal-like phase with fast (∼50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower (∼500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied J_(eff) = 1/2 band undergoes the sharpest upturn at TN, which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr_2IrO_4 to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at T_N in transport and thermodynamic measurements
Observation of spin Coulomb drag in a two-dimensional electron gas
An electron propagating through a solid carries spin angular momentum in
addition to its mass and charge. Of late there has been considerable interest
in developing electronic devices based on the transport of spin, which offer
potential advantages in dissipation, size, and speed over charge-based devices.
However, these advantages bring with them additional complexity. Because each
electron carries a single, fixed value (-e) of charge, the electrical current
carried by a gas of electrons is simply proportional to its total momentum. A
fundamental consequence is that the charge current is not affected by
interactions that conserve total momentum, notably collisions among the
electrons themselves. In contrast, the electron's spin along a given spatial
direction can take on two values, "up" and "down", so that the spin current and
momentum need not be proportional. Although the transport of spin polarization
is not protected by momentum conservation, it has been widely assumed that,
like the charge current, spin current is unaffected by electron-electron (e-e)
interactions. Here we demonstrate experimentally not only that this assumption
is invalid, but that over a broad range of temperature and electron density,
the flow of spin polarization in a two-dimensional gas of electrons is
controlled by the rate of e-e collisions
Selective probing of photo-induced charge and spin dynamics in the bulk and surface of a topological insulator
Topological insulators possess completely different spin-orbit coupled bulk
and surface electronic spectra that are each predicted to exhibit exotic
responses to light. Here we report time-resolved fundamental and second
harmonic optical pump-probe measurements on the topological insulator Bi2Se3 to
independently measure its photo-induced charge and spin dynamics with bulk and
surface selectivity. Our results show that a transient net spin density can be
optically induced in both the bulk and surface, which may drive spin transport
in topological insulators. By utilizing a novel rotational anisotropy analysis
we are able to separately resolve the spin de-polarization, intraband cooling
and interband recombination processes following photo-excitation, which reveal
that spin and charge degrees of freedom relax on very different time scales
owing to strong spin-orbit coupling.Comment: Accepted to Phys. Rev. Let
Band-dependent Quasiparticle Dynamics in Single Crystals of the BaKFeAs Superconductor Revealed by Pump-Probe Spectroscopy
We report on band-dependent quasiparticle dynamics in
BaKFeAs () measured using ultrafast
pump-probe spectroscopy. In the superconducting state, we observe two distinct
relaxation processes: a fast component whose decay rate increases linearly with
excitation density and a slow component with an excitation density independent
decay rate. We argue that these two components reflect the recombination of
quasiparticles in the two hole bands through intraband and interband processes.
We also find that the thermal recombination rate of quasiparticles increases
quadratically with temperature. The temperature and excitation density
dependence of the decays indicates fully gapped hole bands and nodal or very
anisotropic electron bands.Comment: 4 pages, 4 figure
The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors
The condensation of an electron superfluid from a conventional metallic state
at a critical temperature is described well by the BCS theory. In the
underdoped copper-oxides, high-temperature superconductivity condenses instead
from a nonconventional metallic "pseudogap" phase that exhibits a variety of
non-Fermi liquid properties. Recently, it has become clear that a charge
density wave (CDW) phase exists within the pseudogap regime, appearing at a
temperature just above . The near coincidence of and
, as well the coexistence and competition of CDW and superconducting
order below , suggests that they are intimately related. Here we show that
the condensation of the superfluid from this unconventional precursor is
reflected in deviations from the predictions of BSC theory regarding the
recombination rate of quasiparticles. We report a detailed investigation of the
quasiparticle (QP) recombination lifetime, , as a function of
temperature and magnetic field in underdoped HgBaCuO
(Hg-1201) and YBaCuO (YBCO) single crystals by ultrafast
time-resolved reflectivity. We find that exhibits a local
maximum in a small temperature window near that is prominent in
underdoped samples with coexisting charge order and vanishes with application
of a small magnetic field. We explain this unusual, non-BCS behavior by
positing that marks a transition from phase-fluctuating SC/CDW composite
order above to a SC/CDW condensate below. Our results suggest that the
superfluid in underdoped cuprates is a condensate of coherently-mixed
particle-particle and particle-hole pairs
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